1. Field of the Invention
This invention relates to wastewater treatment. The invention especially relates to methods and apparatuses for clarifying mixed liquor in activated sludge processes that are conducted in oxidation ditches of racetrack or loop channel configuration or in conventional complete mix tanks.
2. Review of the Prior Art
Oxidation ditches for continuous aerobic treatment of liquid wastewaters have been used since the early 1950's. They, were developed in Holland by Dr. Ir A. Pasveer, as a variation of the activated sludge process, and were patented in Dutch Pat. No. 87,500 and British Pat. No. 796,438.
Barrier oxidation ditches have been operating since 1977, primarily for treating municipal and poultry processing wastewaters. Barrier oxidation ditches are described in U.S. Pat. No. 4,260,486 of John H. Reid, which is fully incorporated herein by reference. A barrier oxidation ditch comprises an endless channel, a barrier disposed athwart the channel, a vertically mounted pump having an impeller within a draft tube which is vertically mounted at the upstream side of the barrier, a J-shaped discharge duct which is flow-connected to the draft tube and is mounted below the bottom of the channel and below the barrier to provide a discharge on the downstream side thereof, and an aeration means which includes a sparge disposed beneath the impeller, and, if needed, diffusers which are removably mounted so that they introduce additional diffused air at about the bottom of the discharge duct. This barrier oxidation ditch, which pumps all of the circulating mixed liquor past the barrier, is herein described as a total barrier oxidation ditch.
It has further been ascertained that the barrier oxidation ditch of U.S. Pat. No. 4,260,486 creates a differential hydraulic head that is readily measurable when the flow is being pumped through one or more draft tubes and discharge ducts, thereby indicating that there exists a dammed-up momentum in the mixed liquor which is approaching the barrier. It is important to release the energy contained in the dammed-up momentum.
An improved barrier oxidation ditch is disclosed in U.S. Pat. No. 4,278,547 of John H. Reid; it is also incorporated herein by reference. This ditch comprises a barrier having adjustably sized openings and/or gateways therethrough for conserving the momentum of the mixed liquor, a pump means for pumping most of the liquor past the barrier, and an aeration means for aerating the pumped liquor and for selectively aerating the induced-flow liquor passing through the openings so that there is no backmixing of aerated liquor and can be relatively little heterogenous aeration when the aerated pumped liquor is blended, downstream of the barrier, with the induced-flow liquor. This barrier oxidation ditch, which pumps a portion of the circulating mixed liquor past the barrier and provides openings for the remainder to move through the barrier is herein described as a partial barrier oxidation ditch.
One of the major benefits of the barrier oxidation ditch of U.S. Pat. No. 4,260,486 is that the sparge in the downdraft tube provides for introducing diffused air to the mixed liquor at a shallow depth, thereby forming an air-liquor mixture, and then for pumping this mixture downwardly with its impeller into the discharge duct to a sub-channel oxygen-transfer depth, at the lowest portion of the discharge duct, that is well below the channel bottom. This oxygen-transfer depth increases the driving force for transferring oxygen molecules across the films at the gas-liquid interfaces, of the air bubbles. Other additional benefits of great practical importance are: (1) the energy required for downwardly pumping the air-liquor mixture is considerably less than the energy required for downwardly pumping the liquor alone and for separately compressing air to the hydraulic pressure existing at the oxygen-transfer depth; and (2) a very high level of turbulence is provided in the oxygen-transfer zone, measured by brake horsepower/1000 ft.sup.3.
For any aeration system used in transferring oxygen to a particular wastewater, sewage, or mixed liquor, its oxygen transfer efficiency is a function of five major parameters: bubble size, bubble retention time, driving force across the air-liquid interface for the dissolved oxygen, hydrostatic pressure, and degree of turbulence in the oxygen-transfer zone. However, the adjustable gateways through the barrier of U.S. Pat. No. 4,278,547 allow the induced-flow portion of the mixed liquor to pass through the barrier and be aerated at a depth above the channel bottom instead of at the sub-channel depth that is available within a discharge duct, thereby losing at least some of the advantages of retention time, driving force, hydrostatic pressure, and possibly even turbulence.
U.S. Pat. No. 4,455,232 of John H. Reid accordingly discloses a barriered circulator/aerator in the endless channel of a barrier oxidation ditch which provides a directly pumped flow of mixed liquor into a central liquor inlet zone and an induced flow of mixed liquor into a surrounding liquor inlet zone at the inlet of a deep oxygen contact duct which passes beneath the barrier to the discharge channel on the downstream side thereof. It further provides mixing of diffused air with the directly pumped flow and/or the induced flow and then moving the combined air-liquor flows into the deepest portion of the contact duct where point-source pressurized aeration of both flows occurs. Eddy jet diffusers are preferably used for aerating the induced flow. Oxygen transfer efficiencies are obtained that are 1.6 to 2.2 times as great per brake horsepower per hour as that attainable by 100% pumping of the mixed liquor in a total barrier oxidation ditch, as disclosed in U.S. Pat. No. 4,260,486.
This improved barrier oxidation ditch, however, compels 100% of the flow, both pumped and induced, to change direction 90.degree. while moving downwardly and then to change direction 135.degree. while moving beneath the barrier and upwardly. As is well known in hydraulic theory, such 225.degree. of direction changing causes significant energy consumption. It would be desirable to provide a means for passing the liquor from the intake channel to the discharge channel with minimum directional change.
When the air-liquor mixture has reached the lowest portion of the discharge duct, there is also very little time available for oxygen transfer from bubble to liquor across the films of the liquor-gas interface before the liquor/air mixture begins to rise. Yet, it is at this point in passage from the intake channel to the discharge channel that transfer efficiency is highest because of maximum hydrostatic pressure. Another factor of importance is that the microorganisms are in an oxygen-starved condition and avidly utilize the oxygen as fast as it transfers across the liquor-air films into the bulk liquor, so that the bulk liquor cannot become saturated if the MLSS content is reasonably high. An unusually large proportion of the oxygen in the bubbles is accordingly able to transfer across the films into the bulk liquor. It would accordingly be desirable to prolong the bubble retention time at the maximum depth. However, simply lengthening the discharge duct, such as by thickening the barrier, tends to waste the available land area.
An elongated clarifier is described in U.S. Pat. No. 3,788,477 of Love, for use alongside complete-mix tanks. As described in this patent, flow of sludge from the sloping bottom of the clarifier into the adjacent mixing basin is limited by flow control plates and is picked up inside the mixing basin by the downward and inward flow of mixed liquor along the sides and bottom of the mixing basin. Use of sludge return pumps can thereby be avoided.
This clarifier was installed alongside the discharge channel in several barrier oxidation ditches during 1979 and 1980. However, in a barrier oxidation ditch, the flow is translational, not toroidal as in the mixing basin of a complete-mix system. It was consequently discovered that the sludge did not adequately slide out into the discharge channel of a barrier oxidation ditch but had to be pumped out of the clarifier because the turbulent translational flow in the discharge channel created a positive pressure that interferred with automatic discharge of the sludge.
In about 1979, a brochure entitled "Lightnin Treatment System" was published by Mixing Equipment Co., Rochester, N.Y., which schematically showed the integral clarifier of U.S. Pat. No. 3,788,477 disposed downstream of the circulator/aerator in a barrier oxidation ditch and also aligned transversely to the channel so that its upstream side formed the barrier and its downstream or discharge side was athwart the channel and immediately upstream of the terminus of the discharge duct. Moreover, the length of the clarifier, as illustrated, was shorter than its width and was, in fact, the same as the width of the channel. However, such an integral clarifier would, in most cases, be far too small to be combined with a barrier oxidation ditch.
If the size of the clarifier were to be increased by extending the discharge side or outer baffle to a location far down the discharge channel and past the end of the discharge duct, there would be such a pool of violently agitated air moving beneath the clarifier and boiling up past the sludge discharge space of the clarifier that its operation would be impossible. In addition to this discharging difficulty, the bottom of this sideways-extended clarifier would have such a slight slope that settled sludge would not slide downwardly toward the discharge space, whereby its operation would be unsuccessful for this reason as well.
As disclosed in U.S. Pat. No. 4,303,516 of Stensel et al, a rectangularly shaped clarifier is mounted in one or both channels of an oxidation ditch which has a mechanical aerator at one end, as taught in U.S. Pat. No. 3,510,110 of Klein. This clarifier has the same depth as the channel depth, but the channel bottom is excavated beneath the bottom of the clarifier to provide a submerged passage for the mixed liquor. The floor of the clarifier slopes toward the channel sidewall in one embodiment and terminates in a stilling plate that is spaced upwardly from the sidewall to provide a sludge discharge space therebetween. In another embodiment, the floor is horizontally disposed as spaced-apart rectangular plates having rectangular ports therebetween which are athwart the channel, each rectangular port having a shallowly inclined plate along its upstream edge and a steeply inclined plate along its downstream edge. The velocity of the flowing wastewater is increased as it passes beneath the plates so that sludge is drawn from the clarifier through the ports.
This Stensel et al clarifier might perform well if disposed in the return channel, opposite the barrier of a barrier oxidation ditch, or in its intake channel, but if disposed in its discharge channel and close to the barrier, the uprushing air from the discharge duct would enter the clarifier through the discharge ports of either embodiment and disastrously interfere with its operation.
Clarified liquor is typically and generally removed from clarifiers in activated sludge processes by gravity flow. Scum is almost invariably pumped from the scum trough. Settled sludge is removed from the clarifier and recycled to the activated sludge aeration basin by gravity flow followed by pumping, by siphon flow followed by pumping, or by direct pumping. The mixed liquor must be pumped from the aeration basin into the clarifier to create a differential hydraulic head between the surface of the clarified liquor and the surface of the mixed liquor if gravity flow or siphon flow are to operate without the necessity for pumping the return sludge. Less liquid must be pumped, however, if the mixed liquor flows in by gravity and the sludge is returned by pump to the activated sludge treatment apparatus. Pumped flow from the aeration basin to the clarifier is consequently unusual because it requires more power and because the forces generated by a high-speed pumping impeller tend to shear biological flocs, causing sludge settleability to be reduced.
Siphon flow has been used in clarifiers for many years. U.S. Pat. No. 3,494,462 describes a "partial siphon", operating within a circular clarifier having bottom scrapers, in which an air space is maintained above the liquid which is fed into the siphon from the rising pipe to the down pipe, whereby the settled sludge may be perfectly evacuated, especially when the siphon functions as a flow regulator.
A sludge removal system for a clarifier, formed as a rectangular tank and utilizing a plurality of siphons, is described in U.S. Pat. No. 3,333,704. The siphons are supported on a floating carriage. Each siphon comprises a depending pipe of inverted T-form having a horizontally disposed inlet lower branch, a horizontal pipe which is flow connected to the depending pipe, and a U-tube which is flow connected to the horizontal pipe and moves back and forth through a siphon outlet along one long side of the clarifier as the carriage is pulled back and forth within the tank.
Draft tube circulator/aerators have been employed in complete mix basins for several years. Often referred to as a submerged turbine aerator of the axial flow type, a draft tube circulator/aerator has an upflow or downflow impeller which rotates within a draft tube beneath an intake funnel or within the funnel itself. This impeller rotates at 90-100 rpm so that it is a low-speed pump and causes little damage to biological flocs. The draft tube usually extends from several feet below the surface of the liquor to several feet above the bottom of the basin. An aerating device termed an air sparge is disposed below the impeller and provides fine bubbles as the fast-flowing liquor shears the outgoing streams of air.
A pertinent characteristic of both the Love and the Stensel et al clarifiers is that they are integral clarifiers relying upon flowing currents of mixed liquor, which are respectively within an adjacent complete mix tank or oxidation ditch channel, for removing their settled sludges without using a sludge pump for this purpose. Such integral operation is possible because both clarifiers have openings in or near their bottoms which freely allow their sludges to move therethrough. The very practical disadvantage of bottom connections forming such combined systems is that the clarifier and the complete mix tank or the clarifier and the oxidation ditch must be drained together if any repairs are needed to either unit of the combined system. For example, an oxidation ditch having an average volume of 750,000 gallons would be operated in combination with a clarifier of about 25,000 gallons; having to drain the clarifier for repairs would consequently create an enormous additional burden with respect to the oxidation ditch. Therefore, an integral clarifier that has sealed boundary surfaces is needed so that either unit can be drained separately.